Technical Field
The invention relates to the art of disc brakes for heavy-duty vehicles. More particularly, the invention relates to rotors of disc brakes for heavy-duty vehicles. Still more particularly, the invention is directed to a rotor of a disc brake, which includes an inboard attachment to a sleeve, pins that interconnect inboard and outboard discs of the rotor, and an improved metallurgical composition, all of which cooperate to improve the resistance of the rotor to thermal stress created during braking, thereby increasing the performance and the life of the rotor.
Background Art
Disc brake systems for vehicles are well known in the brake art. Such systems operate by forcing a pair of opposing brake pads against a rotor, thereby creating friction between the pads and the rotor to enable slowing and/or stopping of the vehicle. More particularly, a disc brake system includes a plurality of disc brake assemblies, in which each assembly is operatively mounted on or adjacent a wheel end of the vehicle.
Each disc brake assembly includes a carrier, which supports a caliper that is described in greater detail below, and is attached to a torque plate, typically by mechanical fasteners, such as bolts. The torque plate in turn is rigidly connected to an axle of an axle/suspension system of the vehicle, such as by welding. The torque plate resists the torque that is generated during braking, and maintains proper alignment of the carrier and caliper to ensure optimum operation of the components of the brake assembly.
As mentioned above, the carrier supports a caliper, and the caliper is formed with a bore for receiving one or more pistons and an actuator. The actuator typically is a brake air chamber, which is in fluid communication with a compressed air source and activates movement of the piston(s). The caliper also includes a reaction arm that is disposed opposite the piston(s). Each one of a pair of opposing brake pads includes friction material that is mounted on a backing plate, and is seated in the carrier, with one of the pads being adjacent the piston(s) and the other pad being adjacent the reaction arm. Upon actuation by the actuator, the piston(s) and the reaction arm cooperate to control movement of the brake pads.
The rotor includes a disc portion, which is disposed between the brake pads in a manner that allows the friction material of each pad to face a respective surface of the disc portion. The rotor also includes a mounting portion that is adapted for mounting to a respective wheel end assembly of the vehicle by mechanical fasteners, such as bolts. A sleeve typically is integrally formed with and extends between the disc portion and the mounting portion of the rotor. This construction enables the rotor to be rigidly connected to the wheel end assembly, and thus to a respective vehicle wheel.
During vehicle travel, when the vehicle brake system is engaged, compressed air flows to the actuator, which engages movement of the piston(s) and the reaction arm, which in turn forces the friction material of the pads against the disc portion of the rotor, slowing and/or stopping rotation of the rotor, thereby slowing and/or stopping rotation of the vehicle wheel. This forcing of the brake pad friction material against the disc portion of the rotor during braking generates a significant amount of heat, which in turn creates significant thermal stresses on the disc portion of the rotor.
It is well known in the art that significant thermal stresses tend to decrease the performance and the life of the rotor. As a result, it is desirable to employ features in a rotor that are designed to increase air flow and/or dissipate heat. Such features reduce the amount of heat experienced by the disc portion of the rotor during braking, which in turn reduces the thermal stresses on the rotor and improve its performance and life.
A typical disc portion of a prior art rotor includes an inboard disc and an outboard disc, which are spaced apart from one another and are interconnected by a plurality of vanes. More particularly, the inboard disc includes an inboard surface and an outboard surface, and the outboard disc includes an inboard surface and an outboard surface. The inboard surface of the inboard disc is adjacent the friction material of a respective one of the brake pads, and the outboard surface of the outboard disc is adjacent to the friction material of the other one of the brake pads. The outboard surface of the inboard disc and the inboard surface of the outboard disc face or oppose one another, and the vanes are integrally connected to and extend between these surfaces. The vanes are thick ribbon-like structural members that extend radially from the centers or inner perimeters of the respective discs to their outer perimeters. This construction provides a rigid connection between the inboard and outboard discs, while also creating air passages between the discs.
It has been believed in the prior art that such a plurality of air passages between the inboard and outboard discs promoted “pumping” of air between the discs during vehicle travel, thereby providing optimal air flow in the disc portion of the rotor to dissipate heat. However, vane-type connections may not actually optimize air flow between the inboard and outboard discs. As a result, it is desirable to provide means of interconnecting the inboard and outboard discs that increases air flow, and thus heat dissipation, beyond that of vane-type connections.
In addition, for optimum functioning of the brake system, it is desirable for the disc portion of the rotor to be maintained in a vertical orientation, which provides a square, even contact of the inboard surface of the inboard disc with the friction material of its adjacent brake pad, and a square, even contact of the outboard surface of the outboard disc with the friction material of its adjacent brake pad. However, it is known in the art that the heat which is generated from the friction of the brake pads being forced against the disc portion of the rotor causes the disc portion to expand radially. In addition, the heat generated from the friction of the brake pads being forced against the disc portion of the rotor creates a thermal gradient across the rotor sleeve, with the sleeve being hotter in the inboard area that is adjacent the rotor disc portion than the outboard area that is away from the rotor disc portion. This thermal gradient causes the rotor sleeve to expand at the inboard area that is adjacent the rotor disc portion. The radial expansion of the disc portion of the rotor and the expansion of the rotor sleeve adjacent the rotor disc portion causes the outer perimeter of the disc portion to move slightly from its desired vertical orientation and in the direction of the rotor sleeve. The rotor disc portion thereby angles or tilts in the direction of the rotor sleeve, that is, in the outboard direction. Such tilting of the rotor disc portion due to the heat generated from braking is referred to in the art as coning.
When the rotor disc portion experiences coning, the inboard surface of the inboard disc is no longer in square, even contact with the friction material of its adjacent brake pad, and the outboard surface of the outboard disc is no longer in square, even contact with the friction material of its adjacent brake pad. Such uneven contact between the inboard disc and its adjacent brake pad, and the outboard disc and its adjacent brake pad, reduces the efficiency of the brake system. In addition, such uneven contact may create stress areas at each point of uneven contact between the brake pads and the inboard and outboard discs, which may cause the formation of cracks in the rotor disc portion, thereby reducing the life of the rotor. Such uneven contact between the inboard disc and its adjacent brake pad, and the outboard disc and its adjacent brake pad, also undesirably reduces the life of the brake pads.
The coning of the rotor disc portion has been exacerbated in the prior art due to the nature of the connection between the disc portion and the rotor sleeve that has been employed. More particularly, the radial center or inner perimeter of the outboard disc is integrally connected to the rotor sleeve, while the inboard disc is connected to the outboard disc through the above-described vanes. Because the outboard disc includes the connection to the sleeve at its radial center or inner perimeter, a portion of the heat that is encountered by this disc is conveyed away from the disc to the sleeve. As a result, the radial expansion of the outboard disc during braking is less than that of the inboard disc, and this difference in radial expansion between the discs causes the disc portion to cone further in the direction of the sleeve. Such additional coning undesirably increases uneven contact between the inboard disc and its adjacent brake pad, and the outboard disc and its adjacent brake pad, further reducing the efficiency of the brake system and the life of the brake pads. Moreover, an increase in such uneven contact may undesirably increase the stress at each point of uneven contact between the brake pads and the respective inboard and outboard discs, which may in turn increase the formation of cracks in the rotor disc portion, thereby further reducing the life of the rotor. As a result, it is desirable to provide means of connecting the rotor disc portion to the sleeve that reduces coning, and thus optimizes the performance of the brake system, the life of the rotor, and the life of the brake pads.
Moreover, rotors for heavy-duty vehicles have traditionally been formed from cast iron in order to exhibit the strength, hardness and stability required for the braking operation. In the prior art, such rotors have been formed with a high carbon content to maintain the heat transfer properties of the disc portion, which enables the rotor to dissipate heat to reduce the thermal stresses on the rotor, thereby improving the performance and life of the rotor. For example, in the prior art, the carbon content of a rotor has typically been greater than about four (4) weight percent (%) carbon. While such a high carbon content has provided good heat transfer properties, it creates a rotor that is undesirably brittle. When a rotor is brittle, it has little ability to resist high temperature stresses, and as a result, prior art rotors with a high carbon content may undesirably experience crack initiation and propagation. In addition, such a high carbon content undesirably reduces the strength of the rotor at elevated temperatures, which is also referred to in the art as a reduction of the rotor's hot strength. Therefore, it is desirable to provide a rotor that includes a metallurgical composition which desirably maintains the heat transfer properties of the rotor, while decreasing the brittleness and improving the hot strength associated with prior art high-carbon compositions.
As a result, there is a need in the art for a rotor of a disc brake for heavy-duty vehicles that provides means of interconnecting the inboard disc and the outboard disc to increase air flow beyond that of vane-type connections, provides means of connecting the rotor disc portion to the rotor sleeve that reduces coning of the rotor during braking, and includes a metallurgical composition which desirably maintains the heat transfer properties of the rotor while decreasing the brittleness and improving the hot strength of the rotor, all of which improve the resistance of the rotor to thermal stress created during braking, thereby increasing the performance and the life of the rotor, and which also increase the life of the brake pads. The improved disc brake rotor for heavy-duty vehicles of the present invention satisfies these needs, as will be described in detail below.